HR: 13:55h
AN: V33D-02    [Abstracts]
TI: Na8-Fe8-Depth Systematics of MORBs: Implications for Melting Models, Temperature, and Mantle Heterogeneity
AU: * Presnall, D C
EM: d.presnall@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Gudfinnsson, G H
EM: g.gudfinnsson@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
AB: To evaluate the MORB melting model of Klein and Langmuir (1987, 1989) and Langmuir et al. (1992) (hereafter, KL87-92) in a way that is free of inter-laboratory analytical differences, we use the Smithsonian global database of MORB basalt glass compositions to compare unaveraged Na8-Fe8-depth correlations with regional correlations predicted by the KL87-92 model (inverse variation of Na8 vs Fe8 and Fe8 vs depth; covariation of Na8 vs depth). We find that the Kolbeinsey Ridge is the only ridge segment where all three of these predicted correlations occur. Also, the KL87-92 model predicts the formation of mixed high- and low-pressure melts that would show extensive olivine-controlled fractionation at low pressures, but such melts have not been found either at Iceland (a separate collection of 370 basalt glasses all analyzed on the University of Iceland microprobe, some of which are unpublished and used with the permission of the Nordic Volcanological Institute) or anywhere else along the global ridge system. Along the Mid-Atlantic Ridge, mantle heterogeneity at both long and short scales is suggested by (1) a general but uneven increase of Na8 and decrease of Fe8 southward, and (2) an abrupt change in these values at the Charlie Gibbs Fracture Zone. Phase relations in the system CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO (Presnall et al., 2002) show that an inverse Na8-Fe8 correlation (East Pacific Rise) occurs in melts generated from a heterogeneous mantle at roughly constant pressure in the plagioclase-spinel lherzolite transition (0.9-1.5 GPa), whereas covariation of Na8-Fe8 (Mid-Atlantic Ridge) is produced by extraction of melts at different depths along a short (0.9-1.5 GPa) melting column. Thus, MORB systematics at both fast- and slow-spreading ridges are consistent with uniformly low generation temperatures and pressures, but appear to require heterogeneity in both the Pacific and Atlantic mantle and two different melting models. In a carbonate-bearing mantle, trace element signatures of MORBs are expected to be controlled significantly by a deep tail of very low-degree melting (Presnall et al., 2002; Presnall and Gudfinnsson, 2005) and would provide only weak constraints on the depth of major basalt production. For hotspots on or close to ridges (Iceland, Azores, Tristan, Bouvet, Afar, Easter), our results do not support mantle temperatures elevated above those for ridge segments far from hotspots.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005